Tunnels for hydroelectric power stations are used to convey water from a reservoir or an inlet to a power plant, so the potential energy of the water is transformed to electric energy. When the water is flowing though a tunnel, an energy loss induced by friction will always arise. As some of the potential energy is lost, the obtained energy is less than the theoretically obtainable energy. Society has a large and continually increasing energy consumption, leading to a steady requirement of new power plants and consequential environmental damage. It is evident therefore that if energy losses can be minimized then this has a significant advantage for the environment and society.
A lot of tunnels e.g. in Norway are simply mined out with no concrete linings or the like, with the exception of concrete reinforcement of especially weak zones. As the water flows close to the very rough tunnel wall surface in such tunnels, the frictional loss is much larger than it would have been following a smoother surface.
Older tunnels with a relatively small cross section will have a larger head loss compared to newer tunnels. The reason may be that these older towels were dimensioned according to outdated criteria (higher financial interest, higher construction costs and less value of the power) compared to the situation today.
Further, in some situations it will be desirable to increase the water flow through the tunnel, e g. by increasing the power plant""s maximum discharge and/or conveying more water to the power plant, which leads to a disproportionately large head loss.
When constructing new tunnels the head loss is reduced by drilling or mining with a cross section sufficiently large and with a smoothest possible surface. In addition the loss is sometimes reduced by means of smoothing the sole of the tunnel with asphalt or the like. These methods however have technical and economical limitations which result in a head loss that is still significant in most tunnels. If a significant flow increase has been required for a power plant, making a new tunnel parallel with the existing one or providing an enlargement of the existing one, has up until now been the practical solution of such a requirement. Both these solutions involve significant construction costs.
The objective with the present invention is to reduce the frictional loss that appears when water flows through unlined tunnels or other rough walled tunnels. It is a further objective to achieve this with means that are economical, easy to install and which require a minimum of maintenance. Thus, the invention will constitute a method that can contribute to the upgrading of existing power plants close to their maximum theoretical performance, such that unnecessary extensive new constructions of hydroelectric power plants are avoided.
According to a preferred embodiment an entire length of a pipe may be pre-fabricated from a flexible material as one integral, cylindrical unit, tailor-made for the relevant tunnel. This gives the evident benefit that any joining/assembling of different pieces is avoided. A possible disadvantage is that the weight of the pipe may be excessive. A ductile and preferably reinforced fabric may be utilized, like the ones used for oil booms.
With regard to the problems connected with large dimensions and heavy weight, it may, in some connections, be preferred to assemble the pipe from sections of the flexible fabric, prefabricated in suitable length sections. The sections may be pre-shaped in a cylindrical form which are joined section by section to the appropriate length by suitable means, like welding. Alternatively, the sections may be rectangular, in which case one or more rectangular pieces may be first joined (welded) to a cylindrical unit, where after subsequent cylindrical units are attached to one another.
The dimensions are chosen such that when the completed pipe by means of superpressure obtains its intended and nearly cylindrical shape, the pipe fills nearly entirely the regular (cylindrical) part of the tunnel. It is evident that there will be some void between the pipe and the tunnel wall, varying in shape and dimensions depending on the variations of the rough tunnel wall surface. By preferred embodiments which will be described in further detail, this void is also utilized for conveying a limited flow of water, while at the same time maintaining an appropriate superpressure within the pipe so that the pipe is held expanded (cylindrical) during all normal operating conditions.
The tension on the pipe daring normal operation is significant, and it is necessary to prevent the pipe from being displaced in the longitudinal direction of the tunnel by attaching it to the tunnel wall, either at certain intervals or continuously. Tension due to pressure fluctuations however, is avoided, as the (super) pressure within the pipe at all times will he higher than the pressure outside.